1. Field of the Invention
The present invention relates to a method and a device for ascertaining an indicator for the drive-over capability of an object, with the aid of frequency-modulated radar signals from a motor vehicle-mounted radar sensor.
2. Description of the Related Art
Radar sensors are used, for example, in adaptive cruise control systems (ACC systems) for motor vehicles and are used to determine the positions and relative velocities of objects, so within the scope of an adaptive speed control the velocity of the host vehicle may be adapted to the velocity of a preceding vehicle and the distance to the preceding vehicle may be set to a suitable value.
Such a radar sensor has, for example, several channels, to each of which at least one antenna element and one evaluation device are assigned. The high-frequency signals received in the various channels are mixed with a reference frequency and converted into low-frequency signals, also known as intermediate-frequency signals (IF signals). The signals are then evaluated in an electronic evaluation device. For example, the low-frequency signals are digitized with the aid of analog-to-digital converters and then further processed digitally.
Each radar object appears in a channel's frequency spectrum in the form of a peak, the position of which is dependent on the Doppler shift and thus on the relative velocity of the radar object. If the transmission frequency of the radar system is modulated, as for example in the case of a frequency-modulated continuous wave (FMCW) radar system, the position of the peaks is also dependent on the travel time of the radar signals. If in one measuring cycle the transmitted signal has different suitably selected frequency ramps, the relative velocity of the object and the distance to the object may be calculated from the frequencies of the peaks obtained.
Published German patent application document DE 10 2005 024 716 A1 describes a method and a device for detecting and classifying objects, in which the signals received by a radar sensor are evaluated using a comparison with stored characteristic values and the class of the reflecting object is inferred based on the evaluation. One property that may be incorporated in these classes is whether objects are traversable (for example a manhole cover that can be driven over, a beverage can that can be driven over, a road bridge that can be driven under) or are not traversable and cannot be driven over (for example automobiles, trucks, bicycles or motorcycles, guard rails, street signs). A comparison with characteristic patterns takes place, for example by evaluating the spectrum of an object reflection's reflected power, in particular an envelope of the spectrum, and a phase angle.
Published German patent application document DE 10 2010 007 415 A1 describes a radar signal processing method for estimating the height of an object above a reflecting surface, in which a change over time of a distance to the object is determined by a radar system and an intensity modulation of a received signal is ascertained. The method is based on the radar wave interference occurring as a result of reflection from a road surface. The interference patterns, as the vehicle approaches an object, result in intensity modulation in the form of an oscillating backscatter amplitude of the radar signal. The height of the object above the reflecting surface is deduced from the progression of the intensity modulation over a reciprocal distance to the object and/or over the quotient created from a radar height and the distance to the object.